Visibility may be measured in terms of the amount of visible radiation propagated through the atmosphere from distant objects, and the present invention involves the use of such measurements. Daytime visibility, in general, refers to the greatest distance in a given direction at which it is just possible to see and identify a prominent dark object against the sky at the horizon with the unaided eye. If such visibility measurements are made around the entire horizon, they may be resolved, or combined, into a single value of prevailing visibility, which is the greatest horizontal visibility equalled or surpassed throughout half of the horizon circle.
Heretofore, telephotometers have been used for making measurements of visual range in the atmosphere. However, a major disadvantage in the use of such instruments is that a human operator must be present to make the necessary measurements. As a result, continuous measurements of visibility are not practical using such instruments.